1. Field of the Invention
This invention relates to a method of burning pulverous raw material containing lime, such as cement raw meal. The invention also relates to a rotary kiln plant for preheating, at least partially calcining, and burning of such materials.
2. Description of the Prior Art
Calcination of pulverous raw materials such as cement raw meal is to be understood as an expulsion of carbon dioxide (CO.sub.2) from calcium carbonate by an endothermic process (i.e. a process in which heat is absorbed) according to the equation: EQU CaCO.sub.3 .fwdarw. CaO + CO.sub.2
when the raw material is cement raw meal, the aforesaid finishing heat treatment following the calcination is a sintering by which cement clinker is produced. Sintering is an exothermic process characterized by, or formed with, evolution of heat. The sintering therefore only requires a modest supply of heat in order to raise the temperature of the raw material to the sintering temperature and to compensate for losses.
The heat necessary for carrying through the conversion of cement raw meal to cement clinker is usually obtained by burning fuel which together with combustion air, is introduced into a combustion chamber in which the fuel burns successively with the combustion air and forms smoke gas. As a result, the energy contained in the fuel is released for heating the smoke gas to a high temperature. The hot smoke gas is then brought into contact with the raw meal to be heat treated, i.e. preheated, calcined and burned. The heat treatment usually takes place as a continuous process in a rotary kiln with slightly inclining axis. Thus, the lower lying end of the rotary kiln is -- as is usual for rotary kilns -- designed as a combustion chamber.
However, in the manufacture of cement clinker from cement raw meal, by subjecting the raw meal in sequence to preheating, calcination and sintering, the current trend is toward performing the preheating in a separate multi-stage preheater, at least part of the calcination in a calcinator, and the sintering in a rotary kiln. The calcination may be initiated in the lower stages of the preheater and it may not be finished until the material being treated has been fed into the rotary kiln prior to the subsequent sintering. The lowermost stage of the preheater may form the calcinator or act as a calcinator in which a substantial part of the calcination takes place.
U.S. Pat. No. 3,203,681 to Rosa et al. relates to a process wherein heat for carrying through the calcination of preheated cement raw meal derives from hot gases having a temperature higher than the calcination temperature. The gases are produced in a separate chamber and are passed upwardly in a riser column in which the raw material is suspended and entrained by the gases thus produced.
In a related development, commonly assigned U.S. Pat. application Ser. No. 423,436, filed Dec. 10, 1973, now U.S. Pat. No. 3,955,995, dated May 11, 1976, is directed to calcination of pulverous material by mixing preheated raw material intimately with a fuel, capable of producing a combustible gas upon contacting the raw material, providing a gas capable of supporting combustion of the combustible gas thus produced to at least partially calcine the raw material, and separating the treated raw material from the stream of gases. A divisional of Ser. No. 423,436, now U.S. Pat. 3,955,995 is presently pending under Ser. No. 653,025, filed Jan. 28, 1976. Also, commonly assigned U.S. Pat. application Ser. No. 450,291, filed Mar. 12, 1974 relates to a method of heat treating a preheated pulverous raw material wherein the material is accumulated in a lower portion of a calcination chamber and an oxygen-containing gas is introduced into a flame overlying the accumulation.
As can be seen from these examples, in plants in which a major part of the calcination of the material being treated takes place outside the rotary kiln, the fuel and combustion air providing the heat for calcination may be provided in various ways. By way of another example, in copending commonly assigned application Ser. No. 559,705, filed Mar. 19, 1975, the riser pipe of the last preheater stage is designed to act as a calcinator and the hot partly calcined raw material entering the upper material inlet end of the inclined rotary kiln from the last preheater stage is mixed with solid or liquid fuel which, upon meeting the hot raw material, gives off combustible gas. This combustible gas, together with the kiln exit gas, passes to the calcinator in which preheated raw material from the penultimate preheater stage is suspended in the gas mix. Oxygen-containing gas is supplied to the calcinator so that the combustible gas is ignited and the preheated raw material is calcined to the desired extent. As disclosed in that earlier application, the oxygen-containing gas may be supplied to the calcinator through the kiln or as a separate supply by-passing the kiln.
Although it is simpler if the oxygen-containing gas is supplied through the kiln, in such an arrangement more oxygen-containing gas must be supplied to the lower end of the kiln than is necessary for nourishing in the kiln a flame which burns in the kiln for carrying out the heat treatment in the kiln. However, in order that the kiln gases shall have the necessary reaction temperature for the heat treatment in the kiln, extra fuel must be burnt in the kiln to raise the extra volume of oxygen-containing gas to the reaction temperature. However, the kiln reaction requires no greater heat because of the presence of the extra mass of oxygen-containing gas and consequently, the kiln gases leaving the kiln at the upper end of the kiln are at a higher temperature than they would otherwise be and indeed at such a high temperature that they may damage such parts as the upper mouth of the kiln, the adjacent rotary seal and the lower end of the riser pipe which directs the exit gases away from the kiln and serves as a calcinator.
The high temperature of the gases has another no less serious detrimental effect if -- as is often the case -- too great a quantity of compounds having a content of alkali, chlorine and sulphur is present in the cement raw materials. These compounds may, for the sake of convenience, be referred to as alkalis and they are present in the high temperature exit gases in a vaporised state. When these gases sweep the above-mentioned components adjacent to the upper kiln mouth, particularly the lower end of the said riser pipe, the gaseous alkalis will condense on these parts as troublesome solid coatings with embedded raw material.
The same two temperature problems exist if the heat for calcination is provided by burning extra fuel in the lower end of the kiln so that the kiln exit gases have a temperature high enough to perform an adequate calcination after they have left the kiln. I have invented an improved method and rotary kiln plant for preheating, at least partially calcining, and sintering such cement raw meal by supplying the oxygen containing gas through the kiln while avoiding the inherent disadvantages described.